tankless turns on when I turn on the cold water

I recently renovated my house, and installed a Takagi T-K3 tankless water heater. I now find that when I turn on my cold water in the tub, flush my toilet, etc. the tankless water heater fires up briefly, then shuts off. When I shut off the cold water, the heater again fires up briefly, then shuts off. Is this expected, and if not, does anyone know what the problem might be?

I do have a dedicated recirculation line running back to the tankless, but I don't think that is the problem. I also have a check valve on the cold water supply to the tankless as the manufacturer suggests.

Sounds like the check valve from a logical point of view (not always correct!).

The unit is designed to turn on when flow is detected...if the recirculation pump runs constantly, I'm surprised it isn't on all of the time. If the recirc system has control valves that throttle flow down once hot is detected, then the tankless system could turn off.

If I remember, someone mentioned that they recommend a small tank when using recirculation on the tankless to keep cycling down on the unit. Read the installation instructions carefully to evaluate what's needed to make it work and have a decent life.